*Building* with the iOS 12 SDK (e.g. the SDK that is included in Xcode 10 is not supported in LC yet, but it will be supported in the next release.

However, if you want your app to run in iOS 12 devices too (this is different from *building* with the iOS 12 SDK), you can just rebuild the app with LiveCode 9.0.1, using the version of Xcode depending on the version of your Mac OS: